The long term goal of this project has been to understand the role of Sertoli cells in spermatogenesis. To achieve this goal, previous studies focused on gene expression and hormonal response of murine Sertoli cells and the interactions of the somatic cells with the germinal cells. The results of these studies have led to a central hypothesis that attempts to explain how Sertoli cells regulate the maturation of spermatogonia and the entry into spermatogenesis and meiosis. Elements of this testable hypothesis include: A) A major functional role of the Sertoli cells is to regulate the spatial and temporal delivery of retinoic acid to spermatogonia. B) Once delivered, the retinoic acid stimulates the undifferentiated spermatogonia to enter into a differentiation pathway and ultimately meiosis. Induction of the gene stra8 is a sensitive and reliable marker of this process. C) The delivery of retinoic acid via the Sertoli cells is a highly regulated process and is ultimately responsible for the establishment of the cycle of the seminiferous epithelium. The experiments to test this hypothesis are organized into three specific aims: Specific aim 1: Determine the kinetics of stra8 induction and the extent to which this induction in germ cells leads to differentiation in vivo and in vitro. Specific aim 2: Determine if, and how, the flow of retinoic acid through the Sertoli cells to the spermatogonia is regulated and resolve which components of the Sertoli cells are key for inducing stra8 in spermatogonia and initiating spermatogonial differentiation. Specific aim 3: Examine the onset of meiosis and establishment of the cycle of the seminiferous epithelium in models where the retinoid metabolism has been perturbed. The hypothesis requires that gonocytes and spermatogonia do not have access to circulating retinoic acid except that which is delivered by the Sertoli cells. Another requirement is that the delivery of retinoic acid to gonocytes or spermatogonia is sufficient to push these cells into the differentiation pathway. This hypothesis suggests a mechanism for entry into meiosis and thus predicts how the cycle of the seminiferous epithelium can be generated and regulated. This research project could potentially determine the molecular mechanisms that initiate spermatogenesis in the male germline and determine the role of the Sertoli cells in that process. Results from these studies could lead to new fundamental approaches to problems relating to male infertility and contraception. This research project could potentially determine the molecular mechanisms that initiate spermatogenesis in the male germline and determine the role of the Sertoli cells in that process. Results from these studies could lead to new fundamental approaches to problems relating to male infertility and contraception.